FIG. 1 shows an example of a circuit structure for one pixel of an active-type organic EL display device (a pixel circuit). In this structure, a driving thin film transistor (TFT 1) of a P-channel type is connected, via its drain, to an anode of an organic EL element 3, via its source, to a power source line PVdd, and, via its gate, to a source of a selecting thin film transistor (TFT 2) of an N-channel type. The organic EL element 3 is further connected, via its cathode, to a cathode power source CV. The selected TFT 2 is further connected, via its drain, to a data line Data and, via its gate, to a gate line Gate. The gate of the driving TFT 1 is also connected to one end of a holding capacitor C, which is further connected, on its other end, to a capacitor power source line Vsc.
The gate line, which runs in the horizontal direction, is made an H level to thereby turn on the selected TFT 2. With the selected TFT 2 remaining in an ON state, a data signal having a voltage corresponding to a display luminance value is applied to the data line Data, which runs in a vertical direction, so that the data signal is held in the holding capacitor C. Then, the driving TFT 1 supplies a driving current according to the data signal to the organic EL element 3, which is thereby caused to emit light. The amount of light emission is substantially proportional to that of the driving current.
Here, in general, such a voltage Vth that causes a drain current to begin flowing at around a black level of an image is applied to between the gate of the driving TFT 1 and the power source line PVdd, and an image signal is given such an amplitude that can realize a predetermined level of luminance at around a white level.
FIG. 2 shows the relationship between a gate-source voltage Vgs of the driving TFT 1 (a voltage difference between the data line Data and a power source Pvdd) and a current icv flowing in the organic EL element 3 (corresponding to luminance). By determining a data signal such that a voltage Vth is given as a black level voltage and a voltage Vw is given as a white level voltage, color tones for the organic EL element 3 can be appropriately controlled.
The voltage Vth, however, is likely to change due to a change in temperature or external light. That is, upon change in an environment in which the panel is used or generation of heat by the panel itself, image luminance may change and flat black or shallow black may be caused. Moreover, an excessive current may flow into the panel, which may accelerate deterioration of an OLED element.
In view of the above, there has been disclosed a method for detecting total panel current to change contrast and/or a luminance level of an input signal based on a detection result in order to limit a current flowing into the display panel (See Japanese Patent Laid-open Publication No. 2002-251167).
This method, however, cannot compensate for a change in a black and/or white level caused due to an environmental change, as it can only limit a current flowing in the display panel. Thus, an appropriate display cannot be maintained should an environmental condition be changed.